Extensional diapirism overprinted by shortening

Guglielmo, G., Jr., Vendeville, B. C., and Jackson, M. P. A., 1998, Animations of extensional diapirism overprinted by shortening: A BEG hypertext multimedia publication on the Internet at: https://www.beg.utexas.edu/agl/animations/AGL98-MM-006.

ANIMATIONS OF EXTENSIONAL DIAPIRISM OVERPRINTED BY SHORTENING

GUGLIELMO, G., Jr., VENDEVILLE, B. C., and JACKSON, M. P. A.

Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713-8924 USA

ABSTRACT

An animation showing an overhead view of a physical model illustrates the evolution or diapirs that were overprinted by shortening. The deformation includes extensional linear grabens pierced by reactive walls, salt emergence, large diapir overhangs, tight folding and reverse faulting, gentle folding and salt canopies.

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2-D ANIMATION

DESCRIPTION AND INTERPRETATION

This animation links a series of photographs taken during deformation of a scaled physical model (by B. C. Vendeville). The animation illustrates in map view how diapirs that initially formed reactively during regional extension and grew passively could subsequently be deformed by regional shortening. This scenario has been proposed for the Yemeni Red Sea (Heaton et al, 1995), Kwanza Basin (Angola, Duval et al 1993), Nordkapp Basin (Nilsen et al, 1995) and may also be applied to the North Sea and to the lower slope of the Gulf of Mexico (Vendeville and Nilsen, 1995).

The model initially comprises a 3.0 cm-thick tabular layer of viscous transparent silicone overlain by a 1 cm-thick prekinematic sand layer. Illumination is from the right. During the simulation, the left endwall (in the west) moved to the left to produce extension, to the right to produce shortening, or remained stationary to simulate tectonic quiescence. Tabular sand layers deposit during these stages. The model sidewalls (in the north and south) were lubricated to reduce lateral friction and prevent edge effects. The animation lasts 21 seconds and represents ~ 95 hours of the original experiment. The deformation can be described in 7 sequential stages. Extension and shortening rates were 0.5 cm/h except at the last stage when shortening slowed down to 0.1 cm/h. Diagonal streaks visible at some stages are shadows without structural relevance. 3-D visualizations and interpretations can be found in Guglielmo et al (2000).

Stage 1—Initial extension (0–8% time elapsed, yellow-gray surface) - Extensional linear grabens pierced by reactive walls: Two linear grabens trigger underlying reactive walls. The diapir in the central graben remains in the reactive stage, while greater extension in the western graben causes the underlying reactive diapir to evolve into the active stage before emerging at the surface.

Stage 2—First Tectonic quiescence (8–10% time elapsed, gray-brown surface). After deposition of a new sand layer, tectonic deformation has stopped (stationary wall). However, the western diapir continues to rise actively through the thin roof of the western graben due to differential loading. The reactive diapir below the central graben was too deeply buried to pierce actively and emerge.

Stage 3—Second extension (10–56% time elapsed) - Emergence and formation of large overhangs: Salt diapirs extrude to form overhangs at the left-hand graben and from the lubricated sidewalls of the model. The central graben continues to grow because of regional extension but its underlying diapir never reaches the active stage and remains deeply buried. A third graben forms in the east, but the newly formed reactive wall below it never fully penetrates its thick overburden and stops growing.

Further extension and differential loading (yellow -blue surfaces) triggered second-generation diapirs (white and red bulges) that locally pierce, emerge and form large overhangs above the western graben and northern and southern edges of the model. The diapir underlying the central graben continues to rise reactively because of extension. The diapir then pierces actively and emerges along part of the graben.

Stage 4—Second tectonic quiescence (56–60% time elapsed, blue then yellow-gay surfaces): Deposition of new sedimentary layers.

Stage 5—Rapid regional shortening (60–79% time elapsed) - Folding and reverse faulting: Deformation is more intense in the west and in the center. There, the overburden (1) is thinner, (2) is closer to the moving wall, and (3) contains extensive salt overhangs that decrease coupling between strata and thus enhance folding and thrusting. Shortening is accommodated by folding (e.g., blue line in the photo above indicates crest of an anticline), reverse faulting, and thrusting (e.g., red lines), and by squeezing the silicone upward out of the diapir stems and overhangs, forming two third-generation diapirs (A and B).

Stage 6—Third tectonic quiescence (79–81% time elapsed, yellow surfaces) : Additional sand layers covered all structures.

Stage 7—Slow regional shortening (81–100% time elapsed) - Gentle folding and canopies: Further shortening is mainly expressed by broad, gentle folding of the near-surface layers. It enhances expulsion of silicone from buried overhangs and along the model boundaries to extrude at the surface as large allochthonous sheets (dark puddles). Some of these sheets coalesce into canopies.

Cross Sections

The cross section on the right (Jackson et al, 1994) follows the green line, X–X', in the overhead view above. The section was traced from a slice of the model at the end of deformation (C) was reconstructed (D) (by B. C. Vendeville) to show the western (E) and central (F) diapirs during the extensional phase (Stage 2). Salt is red.

Section C shows (1) pinched diapir stems that formed salt welds (G and H), (2) reverse faults above and below salt overhangs (e. g., I and J) that did not connect to each other because the salt effectively decoupled deformation, and (3) salt flows reactivated to create first, second, and third generation diapirs (e.g., K, L and A).

POTENTIAL STRUCTURAL TRAPS FOR HYDROCARBONS

Stage 1: Traps associated to salt-walls would be more common near the walls in the western region.

Stage 2: Periods of tectonic quiescence provide time for hydrocarbon maturation and migration. Continuous diapir rising by buoyancy forces alone could enhance existing diapir-related traps.

Stage 3: Salt flows form overhangs and coalesce into canopies to form extensive seals to any subsalt structures.

Stage 4: Strata draping over salt mounds extruded during Stage 3 could form anticlinal closures and onlap pinchouts. See also Stage 2.

Stage 5: Anticlines (blue line) in the hanging wall of fault bend folds and at crests of buckling folds may form 2-, 3- or 4-way closures. Renewed extension would increase the area sealed by salt canopies.

Stage 6: See Stage 2

Stage 7: Broad, gentle folding could create large anticlinal closures. See also Stage 3.

3-D ANIMATION

This animation shows 3-D structures described in detail by Guglielmo et al (2000)

Samples of visualization and interpretation of extensional diapirs overprinted by compression. Hot colors represent structural highs in Figures 7, 8 and 10, and isochore thicks in Figure 9.

Figures 7 and 8 — Map and oblique views of salt body, showing distinct structural levels and generations of salt, such as the deformed autochthonous salt (A); the first generation of allochthonous salt (B); and the second generation of allochthonous salt (C and D). In the authochthonous salt, the more prominent wall (E) actually rose slower than the shorter wall, which was pinched off to form a secondary weld (F). In the absence of regional topography, younger allochthonous flows (B) were vertically stacked above salt pinch-offs (F). The vergence of fault-bend folds (figure 10) determined the final direction of both the dip of the feeder (G) and the flow of overlying allochthonous salt sheets (C). The collapse of sand layers at the roof of the salt sheet produced depressions in the salt sheet (H, visualized by slicing the digital model along the gray horizontal plane). Pinch-out of underlying stems by shortening produced feederless allochthonous salt sheets (D). The diapir (I) was aborted before extrusion.

Figures 9 and 10 — Isochore map (Figure 9), which represents the vertical thickness of a synkinematic layer (brown and structurally contoured in the oblique view in Figure. 10) deposited on top of an allochthonous salt sheet. Blues represent thins and reds, thicks. To facilitate comparison, the isochore map is overlaid by a black outline of the underlying salt (from Figure 7). The isochore thin (blue and purple in A) is wider than the final shape of the underlying salt flow (black outline in A) because it records the shape of the underlying allochthonous salt during deposition. That showed palinspatically how the weaker salt absorbed much more shortening than the stronger sediments encasing it. In contrast, another thin (B) almost perfectly overlies the underlying allochthonous salt sheet (black outline centered in B) indicating no shortening after the emplacement of the salt in the relatively stable foreland. Palinspastic restorations have helped inference of geologic and hydrocarbon-migration history in salt provinces. However, salt dissolution and salt flow in and out of the section plane make it difficult to determine the shape of salt bodies before deformation, which hampers accurate restorations. Synkinematic isochores (e.g., Figure 9) recorded the former shape of subsequently shortened allochthonous salt, thus improving 2-D and 3-D restorations of salt tectonics.

REFERENCES

Duval, B., C. Cramez, D. D. Schultz-Ela, and Jackson, M. P. A., 1993, Extension, reactive diapirism, salt welding, and contraction at Cegonha, Kwanza Basin, Angola: AAPG Hedberg Research Conference on Salt Tectonics, unpaginated.

Guglielmo, G., Jr., Vendeville, B. C., and Jackson, M. P. A., 2000, 3-D visualization and isochore analysis of extensional diapir overprinted by compression: AAPG Bulletin, v. 84, p. 1095–1108.

Heaton, R. C., M. P. A. Jackson, M. Bamahmoud, and A. S. O. Nani, 1996, Superposed Neogene Extension, Contraction, and Salt Canopy Emplacement in the Yemeni Red Sea, in M. P. A. Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPG memoir, p. 333–.

Jackson, M. P. A., Vendeville, B. C., and D. D. Schultz-Ela, 1994, Salt-related structures: a field guide for geophysicists: The Leading Edge, p. 837–842.

Nilsen, K. T., Vendeville, B. C., and Johansen, J.-T., 1995, Influence of regional tectonics on halokinesis in the Nordkapp basin, Barents Sea, in M. P. A. Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPG Memoir 65, p. 413–436.

Vendeville, B. C., and Nilsen, K. T., 1995, Episodic growth of salt diapirs driven by horizontal shortening: GCSSEPM Foundation 16th Annual Research Conference, Salt, Sediment and Hydrocarbons, p. 285–295.

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ACKNOWLEDGMENTS

This animation was released to the AGL's Consortium in October 1996. This article is published by permission of the Director, Bureau of Economic Geology, The University of Texas at Austin.